Episode 76 – Horse Evolution

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The evolution of horses is not only one of the classic evolutionary stories of the ancient past, it’s also an incredible story of scientific discovery and adjustment. From the dawn of the Age of Mammals to the domestication of modern work animals, we’re taking a trip through Horse Evolution.

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Horses

Horses belong to a group of mammals known as the ungulates, which include all hoofed mammals and their close relatives. This group is split into the artiodactyls (even-toed ungulates), and perissodactyls (odd-toed ungulates).

The order Perissodactyla includes about 17 species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They bear most of their weight on one toe instead of two, hence being called odd-toed ungulates. Image by LittleJerry via Wikimedia Commons.

Today, there are about 10 species of horses, zebras, donkeys, and wild asses. The most abundant equines in our world are domesticated horses, which include over 400 different breeds. In fact, Przewalski’s horse, which lives in Asia, is thought to be the only living horse species that is truly wild, having never been domesticated. Zebras, too, have avoided domestication. All other wild horses are feral populations of domesticated horses

The history of horse domestication is complex, and there are many unanswered questions. Archaeological evidence indicates that the domestication of horses goes back at least 6,000 years ago in Central Asia, but it’s debated whether horses were domesticated once or multiple times.

Przewalski’s horse, or Mongolian wild horse, is often considered the last true wild horse. They are extremely rare and were extinct in the wild until populations were released back into their native home of central Asia. Image by Denis Egan via Wikimedia Commons.

Horse Evolution

Few evolutionary stories are as iconic as that of horses. For a long time, horse evolution was championed as a clear example of the gradual, linear change that Charles Darwin had predicted would appear in the fossil record. It’s been known since the 1800s that early horses were small browsing animals with multiple toes on each foot (as opposed to modern horses’ single toe), and it seemed that over time horses grew in size, took up grazing, and lost all but the middle toe.

This was a common representation of horse evolution in textbooks for decades. Though this is not entirely incorrect, it is an overly simplified view of the true evolutionary history. Image by Mcy jerry via Wikimedia Commons.

Those changes did indeed happen, but it was not nearly so simple as a linear progression. Over millions of years, horses and their ancestors came in many shapes and sizes. Some grew larger and more similar to modern horses, while others remained small, many stuck to browsing in forests, and many even retained their multiple toes.

At one point in Earth’s history, horses were much like gazelles today, with a variety of body types and feeding styles present at the same time and even in the same environment. The modern horse was not the guaranteed outcome out of this evolution. Image by Heinrich Harder via Wikimedia Commons.

Eohippus lived in both North America and Europe during the early Eocene, about 52 million years ago. They stood only about as tall as medium-sized dog. These is often considered among the earliest horse ancestors. Their padded feet had multiple toes: four toes on the forefeet and three toes on the hindfeet. Image by Charles R. Knight via Wikimedia Commons.

The earliest ancestors of modern horses lived during the early Eocene Epoch, around 55 million years ago, and for many millions of years, horse ancestors were relatively small browsers. But during the Oligocene Epoch, when global climate began to grow cooler and drier, and grasslands spread across the globe, horses began to take on more familiar shapes.

Starting around 30 million years ago, we see some horse ancestors developing teeth more adapted for grazing, as well as early examples of toe reduction.

Mesohippus lived during the middle Oligocene of North America. It only had three functional toes on each foot and was a bit bigger that its ancestors at around one meter tall. Image by David Starner via Wikimedia Commons.

Horse diversity reached its peak around 20 million years ago, during the Miocene Epoch. As grasslands continue to spread, we see some horses growing larger body sizes and more grazing-adapted teeth, and some reducing all but the central toe of each foot. At the same time, other horses remained small, many-toed forest dwellers.

Parahippus appeared in the early Miocene Epoch. These early horses had tall, complex-ridged teeth adapted for grazing. Image by Jay Matternes via Wikimedia Commons.

During the Pliocene Epoch, starting around 5 million years ago, as the Earth continues its cooling and drying trend, the first true members of Equus appear, large-bodied and functionally single-toed. These quickly become the dominant group of horses on the planet while the three-toed horses, so common in the Miocene, dwindle until they disappear completely around one million years ago.

Modern horses are well-adapted for grazing with their tall, grinding teeth, and their reduced toes and fused leg bones are specializations for high endurance travel across open environments. Image by Wikipedian Prolific via Wikimedia Commons.

Most of horse evolution takes place in North America, which is also where we get most of our knowledge of the horse fossil record. But at the end of the Pleistocene Epoch, about 12,000-8,000 years ago, horses go extinct in the Americas, along with many other large mammals.

It wasn’t until the 1600s that horses reappeared in their ancestral homeland of North America, reintroduced by Spanish conquistadors.

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 27 – Domestication
• Episode 129 – Rhinos
• Episode 38 – Grass

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